Position emission tomography (PET) studies of Alzheimer's Disease (AD) patients with Delusional Misidentification Syndromes identified reduced glucose metabolism in orbitofrontal, anterior cingulate and temporal brain structures compared to matched AD patients and controls. Delusion formation was independent of cognitive abnormalities. Compared with controls, elderly patients with late onset depression without cognitive impairment, and without "secondary depression", were found by PET to have increased brain glucose consumption in the medial and lateral temporal regions, orbitofrontal and lateral prefrontal regions of the right hemisphere, and decreased consumption in both thalamic nuclei, compared with matched healthy controls. This patient group may represent a distinct depressive subtype reflected in laterality of the brain metabolic abnormalities. Neuropsychiatric rating scales were introduced to quantify behavioral abnormalities in aging and dementia, and to relate these to regional changes in brain metabolism as measured with PET. Behavioral changes likely are a major cause of institutionalization of AD patients.